In general, methods of soldering workpieces for electronic equipment such as printed circuit boards, chip parts, wafer elements, and integrated circuit parts such as BGA's and CSP's (referred to below simply as workpieces) include the flow soldering method in which areas to be soldered are contacted with a wave of molten solder, and the reflow soldering method in which solder paste is applied to areas to be soldered of a workpiece and then the workpiece is heated in a heating apparatus such as a reflow furnace.
In the flow soldering method, a liquid flux is applied to a printed circuit board on which electronic parts are mounted, and after the flux is dried, the substrate is contacted with a wave of molten solder to carry out soldering. However, with the flow soldering method, when portions being soldered are extremely small, not only is it difficult to adhere molten solder thereto, but even if solder adheres, the adhered amount is not consistent, and there are problems with respect to subsequent reliability. Moreover, in the flow soldering method, large equipment is necessary, such as a fluxer which applies flux to a workpiece, a preheater which dries the flux and performs preheating, and a soldering tank which melts solder and produces a wave of molten solder.
In the reflow soldering method, a mask having holes bored in locations coinciding with the areas to be soldered of a printed circuit board is placed atop the printed circuit board, solder paste is then placed atop the mask, and the paste is wiped with a squeegee to fill the holes in the mask with solder paste. The mask is then removed, and the solder paste is applied to areas to be soldered. The printed circuit board is heated in a heating apparatus such as a reflow furnace, and soldering is carried out by melting the solder paste. In the reflow soldering method, solder can be adhered to relatively small areas to be soldered, but this method also has the problem that the adhered amount is not consistent. In addition, in the reflow soldering method, it is necessary to prepare a mask matching the workpiece, and when the workpiece is very small, it is very troublesome to align the holes in the mask with the areas to be soldered of the workpiece. Moreover, in the reflow soldering method, an apparatus for application of the solder paste by printing is always necessary. Not only is this apparatus expensive, but it is necessary to provide a place for its installation.
In light of these problems of the flow soldering method and the reflow soldering method, in recent years, “solder precoats” comprising solder which is previously adhered to areas to be soldered of a workpiece have come to be used. Methods for obtaining a solder precoat include, for example, the plating method (Patent Document 1), the hot leveling method (Patent Document 2), the solder paste method (Patent Document 3), and the solder ball method (Patent Document 4).    Patent Document 1: JP H9-167883 A1    Patent Document 2: JP H11-54890 A1    Patent Document 3: JP H8-307047 A1    Patent Document 4: JP H8-340174 A1